Microlithography (also referred to as photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to light of a certain wavelength. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be imaged onto the photoresist. After exposure the photoresist is developed to produce an image that corresponds to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system that illuminates a field on the mask that may have the shape of a rectangular or curved slit, for example. The apparatus further includes a mask stage for aligning the mask, a projection objective (sometimes also referred to as ‘the lens’) that images the illuminated field on the mask onto the photoresist, and a wafer alignment stage for aligning the wafer coated with the photoresist.
One of the aims in the development of projection exposure apparatus is to be able to lithographically define structures with smaller and smaller dimensions on the wafer. Small structures lead to a high integration density, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus.
Various approaches have been pursued in the past to achieve this aim. One approach is to improve the illumination of the mask. Ideally, the illumination system of a projection exposure apparatus illuminates each point of the field illuminated on the mask with projection light having a well defined spatial and angular irradiance distribution. The term angular irradiance distribution describes how the total light energy of a light bundle, which converges towards a particular point on the mask, is distributed among the various directions of the rays that constitute the light bundle.
The angular irradiance distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be imaged onto the photoresist. For example, relatively large sized features may involve a different angular irradiance distribution than small sized features. The most commonly used angular irradiance distributions are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a pupil surface of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the pupil surface. Thus there is only a small range of angles present in the angular irradiance distribution of the projection light, and all light rays impinge obliquely with similar angles onto the mask.
Different means are known in the art to modify the angular irradiance distribution of the projection light in the mask plane so as to achieve the desired illumination setting. For achieving maximum flexibility in producing different angular irradiance distribution in the mask plane, it has been proposed to use a spatial light modulator including a mirror array for producing a desired irradiance distribution in the pupil surface.
In EP 1 262 836 A1 the mirror array is realized as a micro-electromechanical system (MEMS) including more than 1000 microscopic mirrors. Each of the mirrors can be tilted about two orthogonal tilt axes. Thus projection light incident on such a mirror device can be reflected into almost any desired direction of a hemisphere. A condenser lens arranged between the mirror array and a pupil surface translates the reflection angles produced by the mirrors into locations in the pupil surface. This illumination system makes it possible to illuminate an optical integrator formed by an optical raster element and arranged in or immediately in front of the pupil surface, with a plurality of light spots. Each light spot is associated with one particular mirror and is freely movable across a light entrance surface of the optical integrator by tilting this mirror.
Similar illumination systems using mirror arrays as spatial light modulators are known from US 2006/0087634 A1, U.S. Pat. No. 7,061,582 B2 and WO 2005/026843 A2.
Also the illumination system disclosed in EP 2 146 248 A1 produces light spots on the light entrance facets of the optical integrator. Here the total area of each light spot is much smaller than the area of the light entrance facets. By suitably assembling the light spots in various ways, it is possible to produce different irradiance patterns on the individual light entrance facets. Since these irradiance patterns are individually imaged on a subsequent field plane, the spatial irradiance distribution in this field plane can be simply modified by changing the irradiance patterns on the light entrance facets.
However, this approach involves a mirror array including a very large number of mirrors. This significantly increases the complexity and costs of the mirror array and also of the control systems to control the mirrors. Furthermore, because of diffraction it is difficult to produce light spots that are sufficiently small. In principle it may be considered not to decrease the size of the light spots, but to increase the size of the light entrance facets.
However, this would severely compromise the homogeneity of the illumination, because then a smaller number of individual light bundles are superimposed on the mask.
One approach is to use one or more additional digital mirror arrays in a beam path between the spatial light modulator and the optical integrator, as this is described in US 2013/0114060 A1 and in the yet unpublished patent applications EP 13194135.3 and EP 14155682.5 that have been filed on Nov. 22, 2013 and Feb. 10, 2014, respectively. Since the digital mirror array is imaged on the light entrance facets of the optical integrator, it is possible to produce finely patterned irradiance distributions thereon.
Although the additional digital mirror array provides superior flexibility in producing field-dependent illumination settings, it also adds significantly to the overall system complexity and costs and is therefore warranted only if this flexibility is really involved.
Another approach to produce field-dependent illumination settings is described in US 2013/0114060 A1. Modulator units are arranged in the beam paths of the light bundles that are produced by the optical integrator. The modulator units are configured to variably redistribute, without blocking any light, the spatial and/or angular light distribution of these light bundles. However, the provision of the modulator units in the light paths is quite demanding from a technological point of view if the number of light paths shall be great.